The present invention relates to the field of electronic engine control of internal combustion engines.
Catalytic converters have the ability to reduce nitrogen oxides, and oxidize unburnt hydrocarbons and carbon monoxide that appear in the exhaust gas stream of internal combustion engines. The catalytic converter""s efficiency at removing each pollutant is dependent upon, among other things, the concentration of oxygen present in the exhaust gas. The process that oxidizes unburnt hydrocarbons and carbon monoxides is more efficient when excessive oxygen is present in the exhaust gas. In other words, these two pollutants are readily cleaned by the catalyst when the air/fuel ratio entering the engine is lean. In contrast, the presence of excess oxygen in the catalyst inhibits the efficiency of the nitrogen oxide reduction process. Nitrogen oxides are more efficiently cleaned by the catalyst when the air/fuel ratio entering the engine is rich. Peak efficiency at removing all three pollutants simultaneously usually occurs at one specific air/fuel ratio, or within a small range of air/fuel ratios.
To provide the ideal oxygen concentration within the exhaust gas created by the engine, many engine control designs incorporate two feedback loops from the exhaust gas back to the air/fuel control mechanism. A first feedback loop is created by an air/fuel feedback control module and a first oxygen sensor that samples the oxygen concentration in the exhaust gas upstream from the catalyst. A second feedback loop is created by the air/fuel feedback control module and a second oxygen sensor that samples the oxygen concentration in the exhaust gas downstream from the catalyst. The first feedback loop provides rapid corrections to the air/fuel ratio entering the engine. The second feedback loop provides a bias back into the first feedback loop used to trim the air/fuel ratio to account for aging of the first oxygen sensor and the catalyst.
Difficulties arise in the air/fuel ratio control due to a decreased capability of the catalyst to store oxygen as it gets older. Control systems are often tuned for older catalysts and consequently are inefficient when the catalyst is new.
Several approaches have been taken to introduce a catalyst aging model to account for variations in oxygen storage capability over time. In general, these approaches have involved modifying the air/fuel ratio ramp/jump back waveform, or modifying the first feedback loop to account for the catalyst""s oxygen storage capability as a function of catalyst age. For example, U.S. Pat. No. 5,848,528 issued to Liu on Dec. 15, 1998 discloses a catalyst aging method whereby a proportional gain that is dependent upon the catalyst""s age is used in metering the amount of fuel sprayed into the engine.
Existing catalyst aging compensation methods, however, ignore the effects of the catalyst aging on the second feedback loop. Second feedback loops properly tuned for older catalysts are improperly tuned for newer catalysts, and vice versa. As the oxygen storage capacity of the catalyst decreases, it would be desirable to decrease the rate at which the second feedback loop trims the air/fuel ratio.
The present invention is an air/fuel control system and a method for controlling an air/fuel ratio entering an engine to maintain an oxygen concentration in the exhaust gas downstream from an emission control device at a predetermined value. The present invention includes adjusting the air/fuel ratio in response to a sensor that monitors the exhaust gas downstream from the emission control device. An emission control device model provides an indication of emission control device performance that is used to modify the adjustment to the air/fuel ratio.
The system includes another sensor that monitors the exhaust gas upstream from the emission control device, and a controller in communication with the sensors. The controller issues a command that controls the air/fuel ratio entering the engine. A first feedback loop is established by the upstream sensor and controller to control the air/fuel ratio entering the emission control device. A second feedback loop is created by the downstream sensor and controller to trim the first feedback loop to produce the predetermined oxygen concentration in the exhaust gas downstream from the emission control device.
An emission control device model is provided to modify the second feedback loop. The modification adjusts the feedback trim to account for modeled performances changes in the emission control device.
Engine speed and engine load dependencies may be accounted for by the inclusion of a set point table that controls a sensor set point reference voltage for the downstream sensor. As the engine speed and engine load change, the set point table outputs different sensor set point reference voltages to shift the effective output of the downstream sensor richer or leaner as appropriate.
A learned integral bias table may also be included in the second feedback loop to account for engine speed and engine load dependencies in the exhaust gas oxygen concentration. New entries in the learned integral bias table are inserted using a correction value generated by integrating the downstream sensor""s output while the engine and emission control device are operating under stable conditions. This storage of learned integral bias table entries allows the system to learn and remember changes that occur in the combined characteristics of the sensors and emission control device over long time periods.
Accordingly, it is an object of the present invention to provide a method, and a system implementing the method, for controlling an air/fuel ratio entering an engine in response to a sensor monitoring an exhaust gas downstream from an emission control device, wherein an indication of emission control device performance is used to modify the air/fuel ratio adjustments due to the downstream sensor.
This and other objects will become more apparent from a reading of the detailed specification in conjunction with the drawings.